The present invention relates to low complexity encoders, and more particularly, to low complexity encoders for implementing recommendation G.723.1 of the International Telecommunication Union (ITU-T).
1. Field of the Invention
Lower complexity compressors/decompressors (codecs) may be preferred for some computationally intensive applications. If the complexity of the codec is the bottleneck in a system, complexity reduction is desirable and can result in a significant reduction in millions of instructions per second (MIPS) required to be executed by the encoder.
2. Description of the Related Art
The ITU-T recommendation G.723.1, incorporated herein by reference, relates to dual rate speech coding for multimedia communications transmitting at 5.3 and 6.3 Kbps. The recommendation prescribes certain methods of implementation for each of these transmission rates. The 6.3 Kbps codec has better quality and uses Multi-Phase Maximum Likelihood Quantization (MP-MLQ) for fixed codebook excitation. The 5.3 Kbps codec uses Algebraic Code-Excited Linear Prediction (ACELP). A functional module of the codec which executes these two encoding methods bears almost half of the computational load of the entire G.723.1speech coder. If the methods executed by the functional module are made to have a decreased computational load, the G.723.1 speech coder will have an increased efficiency.
The present invention provides a method of reducing the computational load of a dual rate encoding system, the encoding system being configured to transmit at a first transmission rate using a Multi-Pulse Maximum Likelihood Quantization (MP-MLQ) process or at a second transmission rate using an Algebraic Code-Excited Linear Prediction (ACELP) process, wherein the MP-MLQ process normally searches subframes of excitation signals according to a nominal number of gain scale factors in the execution of quantization steps for encoding the speech signals and the ACELP process normally imposes a first correlation threshold test for entry into an embedded signal processing loop, the method including the step of:
for the MP-MLQ process, reducing the number of gain scale factors employed in the quantization steps, thereby reducing the number of searches, which in turn reduces the computational load; and
for the ACELP process, imposing a second correlation threshold test for entry into a previous signal processing loop in which the embedded signal processing loop is embedded, thereby reducing the number of times the previous signal processing loop and the embedded signal processing loop are entered, which in turn reduces the computational load.
The present invention further provides a dual rate speech coding system having a reduced computational load, the encoding system having Multi-Pulse Maximum Likelihood Quantization (MP-MLQ) processing means for But ting at a first transmission rate and Algebraic Codec-Excited Linear Prediction (ACELP) processing means for transmitting at a second transmission rate, wherein the MP-MLQ processing means normally searches subframes of excitation signals according to a nominal number of gain scale factors in quantization of the speech signals, and the ACELP processing means normally uses a first correlation threshold test for allowing entry into an embedded signal processing loop, wherein:
the MP-MLQ processing means has a reduced number of gain scale actors for reducing the number of searches and thereby reducing the computational load;
the ACELP processing means uses a second correlation threshold test for allowing entry into a previous signal processing loop in which the embedded signal processing loop is embedded, thereby reducing the number of times the previous signal processing loop and the embedded signal processing loop are entered, which in turn reduces the computational load.
Advantageously, embodiments of the invention simplify the ACELP and MP-MLQ methods by reducing the number of recursions, which make less contribution to the metrics. This is achieved by selecting less gain levels or putting an extra threshold to decrease the chance to enter the most computational intensive loops.
Advantageously, the proposed encoder scheme is applicable for both ITU-T recommendations G.723.1 and G.723.1A. For ACELP excitation, further complexity reduction is possible by adjusting the thresholds. This complexity reduction for ACELP excitation is also applicable for G.729 and its annexes.